Responding to the development of recent semiconductor technology and digital technology, industrial apparatuses are making increased use of microcomputers. As a driving source for such apparatuses, a motor capable of digital positioning operations is useful, and hitherto a stepping motor or a servo motor has been used.
However, the stepping motor is not suitable for high-speed rotation since the increase of current is delayed at high-speed, and since the torque it not timely generated because of the influence of iron loss and the time constant of the coil. Particularly when the magnetic pole pitch is made small in order to improve the positional resultion capability, the iron loss influence appears, since the switching frequency per one rotation is increased.
Further, in order to compensate the disadvantages of the stepping motor, there is a method that can be executed where an encoder is provided outside the motor and switching of the conducting phase is carried out at an advanced time corresponding to the motor speed to thereby overcome the delay of the current increasing. However, in this method, there is the disadvantage that the size of the apparatus becomes large because the encoder is provided outside of the motor, and the encoder is expensive.
On the other hand, in the case of the positioning servo system, high-speed running is executed corresponding to digital position instructions, and therefore proper positioning characteristics can be obtained. However, even in such systems there are disadvantages, such as the encoder being expensive and the circuit scale of the control system being large and expensive.
As described above, the stepping motor is incapable of high-speed rotation, and the stepping motor provided with an encoder has the disadvantages of large size and an expensive encoder. Further, the servo motor has the disadvantage of using an expensive control system and an expensive encoder.
In view of the above-mentioned problems, the present invention offers a brushless motor having positioning functions and high-speed rotation characteristics with a size corresponding to the conventional stepping motor size, and considerably simpler circuit construction.